Problem: If $3a + 3b = 5$ and $7x + 3y = -1$, what is $70x + 30y - 6a - 6b$ ?
$= -6a - 6b + 70x + 30y$ $= (-2) \cdot (3a + 3b) + (10) \cdot (7x + 3y)$ $= (-2) \cdot (5) + (10) \cdot (-1)$ $= -10 - 10$ $= -20$